AMD is one of the leading causes of progressive visual impairment in the over-50 population, and eventually leads to irreversible blindness.
27 Traditional diagnosis by ophthalmoscopy has its limitations as a prognostic screening tool, and identification of blood markers has generally been lacking diagnostically. The lack of simple diagnostic tools available for AMD has motivated us to search for diagnostic biomarkers for AMD. The study by Ren et al.
17 has identified three differently expressed miRNAs in a total of 126 AMD patients, miR-27a-3p, miR-29b-3p, and miR-195-5p, which are considered as novel AMD biomarkers.
22 That study also showed that the level of miR-27a is higher in patients with neovascular (wet) AMD compared with the patients with dry AMD.
17 Moreover, an intensive study profiled miRNAs in the vitreous humor and plasma of patients with neovascular AMD; it demonstrated the potential correlation between vitreal and plasma miRNA signatures with neovascular AMD. The results revealed an increase in miR-146a and a decrease in miR-106b and miR-152 in the vitreous humor, which was reproducible in plasma that emphasizes the potential of miRNA biomarkers as diagnostic or prognostic tools.
14
miRNAs play an important role in gene expression in humans. Overexpression or depletion of individual miRNAs is associated with numerous diseases. They are key regulators of several pathophysiological processes, such as immune and inflammatory responses, pathological angiogenesis, and the response to oxidative stress, all of which have been suggested to be associated with AMD pathogenesis and progression.
1 Dysregulation of miRNAs is involved in retinal diseases, including AMD.
28
This study has successfully profiled and validated the differential quantities of miRNAs in serum from AMD patients versus controls. The biomarker potential of three miRNAs (miR-126, miR-19a, and miR-410) was confirmed by qRT-PCR, with significantly increased levels in serum from AMD patients compared with healthy controls. The study reveals differences between the atrophic (dry) and neovascular (wet) types, which can act as potential biomarkers of AMD disease. The identification of circulating biomarkers in AMD has undoubtedly shown great promise for their application in a clinical setting.
Vascular endothelial growth factor (VEGF) is a key factor involved in the development of neovascular AMD by enhancing angiogenesis and increasing vascular permeability.
16 miR-126 was described as one of the angiomiRs, which are specific miRNAs that regulate angiogenesis in vivo, and is involved in retinal vascular development or Corneal neovascularization (CNV).
1,29 miR-126, is located within the
egfl7 gene. Deletion of
miR-126 in mice leads to vessel leaking and hemorrhage, this is owing to loss of vascular integrity, and defects in endothelial cell proliferation, migration, and angiogenesis.
1,30 A recent study has suggested that miR-126 inhibits negative regulators of the VEGF pathway. Therefore miR-126 was investigated as a single miRNA in ischemia-induced retinal neovascularization. miR-126 is downregulated in RF/6A cells (endothelial choroidal cells) during hypoxic-induced conditions by regulating angiogenic growth factors.
31 miR-126 negatively regulates VEGF signaling, which can be mediated by different signaling pathways.
30 Furthermore, knockdown of miR-126 in zebrafish leads to loss of vascular integrity and hemorrhage during embryonic development.
32 This implies that miR-126 is interconnected to other miRNAs that are related to angiogenesis and vasculogenesis.
7,13,28
Chen et al.
33 have shown the underlying mechanism of how miR-410 targets VEGF-A and the potential for treatment of the retinal neovascularization. That study illustrated the mechanism of miR-410 interaction with the 3′-untranslated region (3′-UTR) of the VEGF-A mRNA.
miR-19a belongs to the miR-17∼92 cluster (miRNA cluster encoding miR-17, miR-18a, miR-19a/b, miR-20a, and miR-92a).
33,34 This cluster has an important role in apoptosis and angiogenesis.
35 One of the reports shows that overexpression of the entire miR-17∼92 cluster increased angiogenesis and vascular growth.
36 This suggests the potential proangiogenic effect of miR-19a.
37 Dews et al.
38 also found that miR-19a correlated with decreased expression of several antiangiogenic factors, such as thrombospondin-1 and connective tissue growth factor.
38
Apoptosis occurs in the RPE cells of patients with AMD, leading to secondary photoreceptor cell death.
39 miR-126 was also shown to regulate cell cycle progression and apoptosis.
40 Additional studies have confirmed that miR-126 could increase the apoptotic effects of irradiation stress in vitro, and deregulation of miR-126 expression inhibited cell proliferation, migration and invasion, and induced apoptosis.
41,42
miR-19 (a and b) play an important role in regulating cell proliferation, differentiation, and apoptosis.
43 Despite the fact that AMD is a disease associated with aging and the miR-17∼92 cluster is downregulated during aging and senescence,
44 the results presented show that miR-19a was upregulated in AMD patients compared with controls. This suggests both potential AMD biomarker and a treatment target for atrophic AMD.
Dysregulation of the complement system has a role in the pathogenesis of AMD.
45 It has been demonstrated that miR-126 regulates the innate response, and miR-126 has also been found in the blood of patients with systemic lupus erythematosus, which is a neurodegenerative autoimmune disease.
46 It is reported that C1q downregulates the expression of miR-410, which suggests a potential role of miR-410 in neurodegenerative autoimmune diseases (as AMD is considered by some researchers).
15,41
Analysis of miRNAs, the major contributing in pathophysiological pathways involved in development of AMD (angiogenesis, apoptosis, and complement dysregulation), hints at a complex plausible role for miR-126, miR-19a, and miR-410.
The profiling of miRNAs biomarkers in serum still remains a huge challenge, as we were faced with several limitations in our study, including small sample sizes and biological variation.
15 This study consists of an elderly population, and for the control population it was difficult to find individuals at this age group without underlying conditions, such as diabetes and hypertension. In addition, a small cohort of patients was used in the discovery study, so to overcome this obstacle, a larger cohort was investigated in our validation study. Nevertheless, further validation across a large group of AMD patients and controls is required.